Doing the next action

About 25% of my work is done on an appointment basis (meetings) or on the basis of immediate external demands made by others (another 25%). But, the rest is up to me. But, without the externals pushing me, I have a hard time deciding on what to begin (task initiating). Once I complete an action in a context I seem to lose a lot of momentum if the n/a is not a very similar one or on the same project, and I lose even more if I change contexts. I just read an article about human task switching being very costly. I am wondering if any one has any suggestions on task initiation and on how to maintain focus and mental energy when switching tasks and/or changing contexts (such as off site to on site, office at workplace to home office).
 
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